Resistance welding electronic control circuit



March 2l, 1950 c. B. sTADUM REsIsTANCE WELDING ELECTRONIC CONTROL CIRCUIT 2 Sheets-Sheet l Filed June 25, 1945 March 21, 1950 c. B. s'TADuM RESISTANCE WELDING ELECTRONIC CONTROL CIRCUIT FiledV June 25, 194.5

2 Sheets-Sheet 2 I tg] 129 33 INVENTOR C lare/1c@ B; .Sadufn Patooted Mar. 21, 195o 2,501,358

UNITED STATE-Sl PATENT OFFICE RESISTANCE WELDING ELECTRONIC CONTROL CIRCUIT Clarence B. Stadum, Edgewood, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application June 25, 1945, Serial No. 601,426

1 Claim. (C1. Z50-27) v This invention relates to an electronic control tion thereof, ofthe welding transformer and its system and has particular relation to a control secondary winding connected in the control cirsystem for use with resistance welding apparatus. cuit of the second valve. With the current In electronic controls for resistance welding, through the primary winding of the welding as constructed in accordance with the teachings 5 transformer lagging behind the voltage While the of the prior art, current is supplied from an alrst valve is conductive, as it does in welding systernating-voltage supply source to the welding tems of the character described, the secondary transformer through a pair of inversely conwinding of the auxiliary transformer impresses a nected electric valves. These valves are usually voltage in the control circuit of the second valve discharge valves of the arc-like type, such as igl0 sufficient to render that valve conductive at the nitrons or thyratrons, and are rendered conduobeginning of the next half-period. tive alternately in half periods of the supply A rather complicated and expensive control voltage of opposite polarity throughout the weldapparatus has been employed in the past, to avoid ing period. large transient currents and provide control of In some applications, the welding period is the heat at the weld. The transient currents are initiated at random and extends for a time peavoided by preventing the initiation of the weldriod which is controlled either manually or auing current now until an instant in a half-period tomatically. With such an arrangement and of the supply voltage at least as late as the inwith the current lagging in phase behind the supstant of zero current. The heat at the weld is ply voltage as a result of the inductance of the controlled by causing the valves to be rendered welding load, large transient currents are often conductive at instants in the corresponding haliobtained because of the random initiation of the periOdS which are predeterminabie at Will. The current iow. These transient currents may be instant in a half-period at which a valve is rensufciently large to cause a defective weld. dered conductive determines the R. M. S. cur- Moreover, such an arrangement does not provide `rent `flowing through the welding transformer any control of the heat at the weld, for the valves and, consequently, determines the heat develconduct the maximum current available to the oped at the weld. It is to be noted, however, that welding transformer. In addition, the welding the usual follow-up Circuit CarmOt be employed period, unless accurately controlled, may extend With apparatus DI'OViding heat 001115101 0f this over an odd number of half periods of the source nature because the second or follow-up Valve a1- voltage tending to cause saturation of the weld- `Ways @Onducts the maximum current available. ing transformer with correspondingly undesira- In Such Cases, Saturation 0f the transformer is b1e Welding operations, avoided by other means such as the use of a com- Saturation of the welding transformer has Dliated timing System. sometimes been avoided in the past by providing It is an Object 0f my invention i0 PFOVide a what is known as a follow-up circuit in the oonnew and improved electronic apparatus for oontrol system. In systems using such prior followtrolling the elleisization 0f a load circuit.

up circuits, the welding period is initiated at will Another Object 0f my invention iS t0 provide a whereupon the first of the inversely connected I16W 'and Simplified COIiIOl System fOi use in revalves is rendered conductive in a half-period of SiStanCe W1C111g the supply voltage of one polarity. This nrst A further object of mv invention is to provide valve continues to he rendered oonuuotive in each a new and improved apparatus for controlling enhalf-period of the same polarity throughout the ergization of a load from an alternating-Current wading period, The second or follow-up valve source in which large transients are avoided. is rendered conductive in each half-period of A further object of my invention is to provide opposite polarity but only as a result of conduca new and improved apparatus for controlling tion of the rst valve in the immediately preenergization of a load transformer from an alceding half-period. Moreover, the second valve ternating-current source in which saturation of is always rendered conductive in the half-period the transformer is avoided. following a conductive half-period of the first Still another object of my invention is to provalve regardless of the action of the automatic or vide a new and improved apparatus for controlmanual timing control. ling energiaation of a load from an alternating- To accomplish such follow-up operation, an current source in which the R. M. S. load current auxiliary transformer has its primary winding is predeterminable at will. connected across the primary winding, or a pori5 More specifically, it is an object of my invention to provide a new and simplified apparatus for controlling the energization of a welding transformer from an alternating-current source in which large transients and saturation of the transformer are avoided, and the R. M. ,S. current supplied to the transformer is predeterminable at will.

In accordance with my invention, I provide a control system in which the rst of a pair of inversely connected valves is rendered conductive in half-periods of the source voltage of one polarity at an instant predeterminable yat will. A control circuit for the second valve is arranged to maintain the valve normally nonconductive, but to render it conductive at an instant predeterminable at will in a half-period of the source voltage of opposite polarity provided the first valve was conductive in the preceding half-period. In the preferred embodiments illustrated, this is accomplished by a novel follow-up circuit.

The features of my invention, which I consider novel, are set forth with more particularity in the appended claim. The invention itself, however, with respect to the details thereof, together with additional objects and advantages, may be better understood from the following description of an embodiment of my invention and a modification thereof with reference to the accompanying drawings, in which Figure 1 is a schematic diagram illustrating the preferred embodiment of my invention;

Fig. 2 is a series of curves illustrating the operations of the apparatus of Figure l;

Fig. 3 is a modification of the apparatus shown in Figure 1, and

Fig. 4 is a series of curves illustrating the operation of the apparatus of Fig. 3.

As shown in Figure 1 of the drawings, the primary winding 5 of a welding transformer 'i has one terminal B connected directly to one of a pair of alternating-voltage supply lines Il and 3. The other terminal l5 of primary winding 5 is connected to the other supply line i3 through a pair of inversely connected electric discharge valves i? and I9 of the arc-like type, preferably ignitrons. Because of the inverse connection, the anode 2| of the first ignitron is connected to the cathode 23 of the second ignitron i9, and the anode 25 of the second ignitron i9 is connected to the cathode 21 of the first ignitron Consequently, the first ignitron Vi may conduct current only in a direction from the second supply line |3 through the primary Winding 5 to the first supply line H, while the second ignitron i9 may conduct current only in the opposite direction. The secondary winding 29 of the welding transformer is connected in circuit with a pair of welding electrodes 3i and 33 and the material to be welded which is engaged therebetween.

An electric discharge valve 31 of the arc-like type, such as a thyratron, hereinafter designated as the first firing' valve, has its anode 39 and cathode 4| connected between the anode 2| and ignition electrode d3 of the first ignitron II. Another electric discharge valve 65 of the arc-like type, such as a thyratron, hereinafter designated as the second firing valve, has its anode 41 and cathode i9 connected between the anode V25 and ignition electrode 5| of the second ignitron I9. As is well known, when either of these firing valves 37 and 45 is rendered conductive, current ows through the ignition electrode of the corresponding ignitron to render that ignitron conductive. When an ignitron becomes conductive, it

`4 effectively short-circuits the corresponding firing valve, causing it to become nonconductive.

A control circuit is provided for the first firing valve 3T which extends from the grid 53 thereof through a grid resistor 55, a biasing resistor 51 and a third resistor 5c to the cathode 4|. The third resistor 59 is short-oircuited through a normally closed contact 6| of a relay E3, and so has no eect upon the firing valve 31. The biasing resistor 57 has a biasing voltage developed thereacross by current supplied through a rectifier B5 and an auxiliary transformer El' energized from the supply lines and I3. A lter capacitor B9 is connected in parallel with the biasing resistor 5?. The voltage appearing across the biasing resistor 5i is of such polarity and magnitude as to maintain the grid 53 of the first firing valve 3'! sufficiently negative with respect to the cathode 4| to prevent the valve from becoming conductive.

A phase-shifting circuit is provided comprising a capacitor il a potentiometer 'i3 and a variable resistor i5 connected in series in the order named across the terminals of a secondary winding of the auxiliary transformer 5i. The primary winding 79 of an impulse transformer Si is connected in series with a resistor 83 from the junction point between the capacitor 'Ii and the potentiometer 13 and an intermediate tap 85 on the secondary winding T1. A pair of secondary windings 81 and 89 is provided for the impulse transformer 8i, and one of these windings B7 is connected across the third resistor 59 in the control circuit of the first :dring valve Si. As a result of the phaseshifting arrangement, va voltage impulse is impressed across the third resistor 59 at a predetermined instant in each half-period of the supply voltage. The polarity and magnitude of this voltage impulse is such that when contacts Si are open it over-comes the biasing voltage across the biasing resictor 5l and renders the rst firing valve 31 conductive in each half-period of the supply voltage in which the anode 3i) is positive with respect to the cathode iii. The instant in a half-period at which the voltage impulse occurs is determined by the adjustment of the variable resistor 'i5 and the potentiometer i3 in the phaseshifting circuit.

The energizing coil il! of the relay 63 is connected across a portion of the secondary Winding 1'1 of the auxiliary transformer 6I through the originally open contacts 93 of. a time delay relay 95 and the originally open push-button switch 97. The energizing coil 99 of the time delay relay 95 is connected directly across a portion of the 'secondary winding 7i, but is arranged to effect closure of the contacts 93 only after a predetermined time delay, of the order of five minutes, to sure adequate heating of the cathodes of the firing valves before operation of the circuit is initiated by operation of the push-button switch.

The control circuit of the second firing valve 45 may be traced from the grid mi thereof through a grid resistor |63' and a conductor |05 to the negative terminal |071' of a biasing capacitor i139 which is charged from the secondary winding TI of the auxiliary transformer 6i through a rectifier H I. The control circuit continues from the positive terminal H3 of the capacitor |09 to the center tap of the secondary winding 17, and from the lower end of the secondary winding through a second resistor I l5 and a third resistor v| l1 to the cathode 49.

The voltage across the biasing capacitor |09 tends to maintain the second firing valve 45 nonconductive. However, the second secondary Y.of the auxiliary transformer 61.

`capacitor |09 to the cathode |23.

first firing valve 31. ence, it is assumed that the base line |41 of curves Winding 89 of the impulse transformer 8| is connected across the third resistor ||1 in the con- ,.-trol circuit of the second firing valve 45. As a result, a voltage impulse is impressed across the .third resistor ||1 at predetermined instants in .each half-period which is sufficient to overcome the biasing voltage of the capacitor |09 and tends to render the second ring valve 45 conductive at the predetermined instant in a half-period in which the anode 41 is positive with respect tothe `cathode 49. But an alternating voltage is also -impressed in the control circuit of the second firing valve 45 through the secondary winding 11 The phase vof this alternating voltage in the control circuit is such that the voltage tends to make the grid highly negative during the half-periods of the supply voltage in which the anode 41 is positive with respect to the cathode 49. The alternating voltage thus impressed in the control circuit is `sufficient to prevent the voltage impulse across the third resistor I |1 from rendering the second firing valve conductive.

An auxiliary electric discharge valve |9 of the arc-like type, such as a thyratron, has its anode |2| and cathode |23 connected across the portion of the secondary winding 11 of the auxiliary transformer 61 which is included in the control circuit of the second firing valve 45. The convnection is such that the auxiliary valve I I9, when conductive, effectively prevents the alternating voltage of secondary winding 11 from being im- .tpressed in the control circuit of the second firing Vit nonconductive.

The secondary winding |3| of another auxiliary .transformer |33 is connected across the second resistor |29 in the control circuit of the auxiliary valve I9. The primary Winding |35 of this auxiliary transformer |33 is connected across a resistor|31 in parallel with the primary winding ofthe welding transformer 1. The purpose of the auxiliary transformer |33 is to supply a volt- --age in` the control circuit of the auxiliary valve H9 sufcient to overcome the bias voltage of cafpacitor |09 and render the valve conductive.

The operation of the system illustrated in Fig. 1 may be better understood by reference to the curves of Fig. 2. In Fig. 2, there is shown a se- .'ries of curves representing either current or voltage plotted against time with identical time bases being employed with all of the curves. Curve |39 represents the current through the primary Winding 5 of the Welding transformer 1 with the portion above the base line |4| representing curi rent through valve |1 and the portion below representing' current through valve I9. Curve |43 represents the supply voltage with respect to the anode 39 of the first ring valve 31, While curve |45 represents the grid-to-cathode voltage of the For purposes of conveni- |43 and |45 also represents the critical value of the grid-to-cathode voltage of the rst firing valve 31 so that the valve is rendered conductive when 'I the actual grid-to-cathode voltage becomes more positive than the Acritical Value. Curve |49 represents the supply.v voltage with Yrespect` to..y the anode |2| of the auxiliary valve H9, while curve |5| represents the gridato-cathode voltage of that valve. The base line |53 of curves |49 and |5 also represents the critical value of thegrldto-cathode voltage of the auxiliary valve. `The supply voltage with respect to the anode 4l4 of the second firing valve 45 is representedbythe curve |55 While the grid-to-cathode voltage is represented by curve |51, and the component of the grid-torcathode'voltage supplied by the biasing capacitor |09 is represented by dotted curve |59. The base line |6| of curves |55, |51 and |59 also represents the critical value of the gridtocathode voltage of the second ring valve. Curve |63 on base line |65 represents the secondary winding voltage of transformer |33 with respect to the grid |25 of auxiliary Valve I9.

With the apparatus of Figure 1 in the condition illustrated, the voltage and current relations are as shown beginning at point |61 in Fig. 2. The supply voltages represented by curves |43, |49 and |55 are being impressed between the anodes and cathodes of the corresponding valves. The

Vgrid-to-cathode voltage curve |45 for the first firing valve 31 remains at a substantially constant, negative value determined bythe voltage .across the second resistor 51 in the valves control circuit. The grid-to-cathode voltage curve l|5| for the auxiliary valve ||9 also remains at a substantially constant negative Value determined by the voltage on the biasing capacitor |09 in the valves control circuit. The grid-to-cathode voltage curve |51 for the second firing valve 45 yis made up at this time of the substantially constant direct current biasing voltage of the capaci- -tor |09 (curve |59) on which is superimposed an alternating voltage from the secondary Winding 11 of the auxiliary transformer 61 and the voltage impulse across the third resistor |1 supplied through the phase-shifting circuit, which impulse results in peaks |69 on curve |51. It is to be noted `that When the supply voltage curve |55 indicates the anode of the second firing valve is positive, the grid-to-cathode voltage curve |51 is maintained less positive than the critical value. Consequently, the second firing valve 45 is not rendered conductive.

Let it be assumed that the push-button switch 91 is closed at a time represented by the position of the vertical dotted line I1 Assuming the time delay relay 95 is energized, the relay 63 is then energized upon closure of the push-button' switch. The contacts 6| or relay 63 lopen the short-,circuit across the third resistor 59 in kthe control circuit of the first firing valve 31.v vAs a result, Voltage impulses across the resistor 59 are impressed in the control circuit causing peaksin the` grid-tocathode voltage curve as shown at |13. In the half-period of the supply voltage curve |43 in which the anode of the first firing A valve 31 is positive, the voltage impulse across the |31.in,parallel with the primary Winding 5 of the transformer 1 and the voltage developedtherecircuit.

facrossis supplied through the auxiliary transfforxner |33 to the Acontrol circuit of the auxiliary valve Il l9. The voltage so impressed is shown by curve |63, and causes the grid-to-cathode voltage curve 15|- for the auxiliary valve H9 to become moreppositive t'han the critical value curve |53 thereof at an instant represented by the dotted vertical line H1. Consequently, the auxiliary valve H9 is rendered conductive at that instant.

When the auxiliary valve l I9 becomes conductive, the alternating-voltage component of the grid-fto-cathode voltage curve |51 for the second ring valve 45 is substantially eliminated. Consequently, the next grid-to-cathode voltage peak l69`causes the gridtocathode voltage curve 151 to rise above the critical value curve 16| and the second ring valve 45 is rendered conductive to render the second ignitron i9 conductive at the instant represented by the dotted vertical line 18. This, of course, occurs in the half-period of the supply voltage immediately following the half-period in which the iirst ignitron l1 was rendered conductive.

lBefore the voltage impulse to render the second ignitron conductive occurs, the current through the welding transformer reaches zero, as at the time represented by dotted vertical line |19, 'and the rst ignitron becomes nonconductive. The grid-to-cathode voltage curve |51 of the auxiliary valve then drops to the value of the biasing voltage supplied by the capacitor 100.

However, the auxiliary valve l i9 has already been rendered conductive and since the valve is of the arc-like type, the grid has lost control. Consefluently, the auxiliary valve I I9 continues to conduct until its anode-to-cathode voltage drops substantially to zero and the second firing valve 45, and, therefore, the second ignitron 19, is rendered conductive as previously described to supply 'current through the second ignitron to the welding transformer. Upon the current dropping substantially to zero, the second ignitron becomes nonconductive. If the push-button switch 51 remains closed, the first ring valve 31 is again rendered conductive at the instant of the voltage impulse from the phase-shifting circuit in each half-period in which the anode of the valve is positive. Each time the nrst ignitron conducts current, the auxiliary valve is rendered conductive so that the second ring valve and the second ignitron are, in turn, rendered conductive in i the following half-period. Consequently, if the push-button switch be opened at some instant Aafter the iirst ignitron is rendered conductive, represented by the dotted vertical line |81, current flows in the welding transformer until a 4time represented by the dotted vertical line 183.

As previously indicated, the instant at which the Vvoltage impulses from the phase-shifting circuit occur is determined by the adjustment of the variable resistor 15 and the potentiometer 13. The potentiometer 13 is preferably calibrated in 'terms of per cent of heat to be developed, while thevariable resistor 15 is lcalibrated in terms of power factor of the welding transformerv supply Prior to operation of the system, the variable resistor 15 is adjusted so that the voltage impulse with a 100%heat setting on the potentiometer 13 occurs at the instant in a halfperiod at which the load current would pass 'through zero for the particular power factor involved. The potentiometer 13 is then adjusted at will to delay the voltage impulse even further -by any desired amount to effect a supply .of a de- `aired average current.

It, therefore, becomes apparent that large transient currents are avoided even with a heat setting of the potentiometer; the R. M. S. current supplied may be controlled at will by adjusting the potentiometer; and saturation of the welding transformer is avoided by the followup characteristic of the control system.

It is to be understood that while a system is illustrated as employing ignitrons in the supply circuit for the welding transformer which are rendered conductive by thyratrons, the ignitrons may be eliminated in some applications with the thyratrcns no longer functioning as the firing valves but actually carrying the transformer supply current.

The modified circuit shown in Fig. 3 is very similar to that shown in Figure l, and the same reference characters have been applied to corresponding elements. The major difference in the circuit of Fig. 3 from that of Figure 1 is that the latter has a self-biasing arrangement for the first and second firing valves 31 and 45. To permit operation of this self-biasing arrangement, the relay 63, which is energized by the push-button switch 91, has normally open contacts 20i in the anode circuit of the rst firing valve, and the contacts 6| short-circuitng the third resistor 50 in the control circuit of the rst iiring valve of Figure l is eliminated in Fig. 3.

The control circuit of the first firing valve 31 in Fig. 3 may be traced from the grid 53 thereof through the grid resistor 55, a capacitor 205 shunted by a resistor 201 and the second and third resistors 51 and 59 to the cathode 4l. The third resistor 59 has impressed thereacross voltage impulses as described in connection with Figure l. The second resistor 51 has a voltage developed thereacross by current supplied from the auxiliary transformer 61, but the rectifier in the supply circuit of Figure 1 is not included in Fig. 3 so that in the latter circuit the voltage across the second resistor 51 is alternating. During the half-periods of alternating voltage across the second resistor 51 in which the grid 53 is positive, current will iiow from the resistor 51 through the capacitor 205, grid resistor 55, grid 53 and cathode 4l and third resistor 59. The rectifying action of the grid-cathode circuit prevents the iiow of current in the opposite direction. During the flow of current in the grid-cathode circuit, the capacitor 205 is charged in a direction tending to make the grid 53 negative with respect to the cathode 4l. In the following halfperiod, the capacitor 205 discharges gradually through the resistor 201 in shunt therewith. The voltage appearing across the capacitor 205 makes it certain that the first valve is not rendered conductive except by the voltage impulse on the resistor 59.

The control circuit of the second firing valve 45 may be traced from the grid 10| thereof through a grid resistor 03, a capacitor 208 shunted by a resistor 2H to the intermediate tap 85 of the secondary winding 11 of the auxiliary transformer 61, and from the lower end ofthe secondary winding 11 through the second resistor H5 and the third resistor H1 to the cathode 49. Since the alternating-voltage component supplied through the secondary winding 11 in the control circuit of the second firing valve 45 causes the grid 101 to become positive with respect to the cathode 49 during the half-periods of the supply voltage in which the anode -41 of the second firing valve 45 is negative, the capacitor 209 in the control circuit is charged by current flow in the grid-cathode circuit simivlarly to the charging of the capacitor 205 at the control circuit of the rst ring valve 31. The voltage on capacitor 209 serves the same function in the Fig. 3 circuit as the voltage across the capacitor |09 in Figure 1.

The control circuit of the auxiliary valve I I 9 in Fig. 3 extends from the grid |25 thereof through the grid resistor |21, the second resistor |29 energized from the second auxiliary transformer |33 and a third resistor 2|3 to the cathode |23. The third resistor 2|3 is connected in series with another capacitor 2I4, the energizing coil 9| of the relay 63, the contacts 93 of the time delay relay 95 and the push-button switch 91 across a portion of the secondary Winding 11 of the auxiliary transformer 61. Consequently, an alternating voltage appears across the third resistor 2|3 which is a little more than 180 out of phase with the anode-tocathode potential.

The operation of the circuit of Fig. 3 may be better understood from the curves shown in Fig. 4, many of which are similar to curves of Fig. 2 and bear the saine reference characters. In Fig. 4, however, the grid-to-cathode voltage of the rst i'iring valve 31 is represented by curve 2I5 with the component thereof appearing across capacitor 205 being shown by curve 2I1. The grid-to-cathode voltage of the auxiliary valve I I9 is illustrated by curve 2I9. Curve 22| represents the grid-to-cathode voltage of the second firing valve with the component thereof appearing across capacitor 209 being shown as curve 223.

The grid-to-cathode voltage of the first firing valve 31 as shown in curve 2I5 is made up of the voltage across the capacitor 205 upon which is superimposed the alternating Voltage across the resistor 51 and the voltage impulses across the third resistor 59 which appears as peaks 225 on curve 2I5. It will be noted that the grid-tocathode voltage curve 2I5 remains substantially constant in the portion thereof in which the grid is positive as the grid-cathode current is then owing. During this time the capacitor 205 is charged as shown along portion 221 of the dotted curve 2I1, and when the grid becomes negative, the capacitor 205 is discharged along portion 229 as shown. While the grid-to-cathode Voltage becomes positive at the instant of the voltage peaks 225, the rst ring valve is notv rendered conductive originally because its anode circuit is open at contacts 20 I.

The grid-to-cathode voltage of the second iiring valve 45 as shown in curve 22| is made up of the voltage across the capacitor 209 plus the alternating `voltage supplied from the secondary winding 11 and the voltage impulses across the resistor II1 which appear as peaks 23| on curve 22|.

Let it be assumed that the push-button switch is closed at the time represented by the dotted vertical line 232. The relay 63 is then energized completing the anode circuit of the iirst firing valve 31. The first firing valve 31 is then rendered conductive at the instant of the next voltage peak 225 in the half-period in which the anode of the valve is positive as represented by the vertical dotted line 233. The first ignitron I1 is accordingly rendered conductive and Welding current ows as illustrated in curve I 39. The voltage impressed in the control circuit of the auxiliary valve I I9 by the second auxiliary transformer |33 as a result of the current flow through the rst ignitron is shown in curve |63 and causes the grid-to-cathode voltage curve 2I9 to become positive at a time represented by the vertical to render the second ignitron I9 conductive.- the first ignitron I1 becomes nonconductive at a time represented by the dotted vertical line 239 and in the manner described in connection with Fig. 2.

So long as'the push-button switch 91 remains closed, the rst firing valve 31 andthe rst ignitron I1 continue to be rendered conductive in alternating half-periods and the second firing valve 45 and second ignitron I9 are always rendered conductive in a half-period following the half-period in which the first ignitron I1 is conductive.

While I have shown and described specific embodiments of my invention. I am aware that many modifications thereof are possible without departing from the spirit of the invention. It, accordingly, is not my intention to limit my invention to the specific embodiments shown and described.

I claim as my invention:

For use in supplying current through a load from a pair of alternating voltage supply lines, the combination comprising a pair of inversely connected valves of the arc-like type interposed in circuit with said load across said lines, a rst control circuit having output connections to the first of said valves for rendering the rst of said valves conductive in a half-period of the supply voltage of one polarity, a second control circuit having output connections connected to the second-valve to control its conductivity, the net potential of said second control circuit across said last-named output connections, relative to an electrical reference point, being normally more negative than a predetermined critical magnitude which said net potential must exceed algebraically to render said second valve conductive, a capacitor, an auxiliary transformer energized from said lines and having a secondary winding with one end connected to one side of the capacitor, a rectifier connected to conduct current from the other side of the capacitor to an intermediate point on said secondary winding whereby the capacitor is charged, a peaking transformer energized from said lines and having a secondary Winding, said second control circuit including the capacitor and the secondary windings of said transformers, said secondary winding of said peaking transformer being effective to impress across said output connections of said second control circuit at a predetermined instant during each half-period of opposite polarity, impulses of short duration compared to the half-periods of the voltage derived from said lines which are positive relative to said reference point, and said capacitor and said transformer being so connected to said last-named output connections that they impress across said last-named output connections during each said half-period of opposite polarity, potentials which are negative relative to said reference point the resultant voltage so impressed being less positive than said critical magnitude, an auxiliary valve of the arc-like type having an anode and cathode connected in a shunt circuit across a part of the secondary winding of the auxiliary transformer and having a control electrode, and circuit means connecting said electrode to the plate of said capacitor which 2,501,368 `11 is electrically negative relative to said point REFERENCES CITED whereby the auxiliary valve is normally non-con- .Y s ductive, said last-named circuit means .including lrghf fgvgerences are of record m the means responsive to load .current in a half-period oi said one polarity to Vrender the .auxiliary valve 5 UNITED STATES PATENTS conductive in the next succeed-ing half-period yof Number Name Date opposite polarity reducing the absolute magn- 1 970 515 Lord Aug 141934 tude of the negative voltage (relative to said ref- 2126398 Knowg T* Aug 9 1938 erence point) impressed across said last-,named 2169023 Dawson Aug 8 1939 output connections to a magnitude such .that the 1o 2270799 Guuksel; an '20 1942 -net voltage across said last-named output con 2283719 Bvens "Ma 19 1942 nections is above .the critical magnitude during 23311124 LivingsgO- O' 5 1943 said voltage impulses of short duration.

CLARENCE B. STADUM. 

